(1) Field of the Invention
The present invention relates to a counter balancing type crane having a counterweight car located behind a crane body.
(2) Description of the Prior Art
A counterbalancing type crane in which, in order to hoist a large load, a counterweight carriage car is disposed behind a crane body in addition to a counterweight attached to the crane body, has heretofore been well known as disclosed in U.S. Pat. No. 3,842,984. This conventional counter balancing type crane is of such a structure as schematically shown in FIG. 10. As shown therein, a crane body 101 comprises a travelling unit 103 and a swing unit 104 disposed on the travelling unit 103; a boom 105 and mast 106 are attached tiltably to the crane body 101; a first guy line 107 for supporting the boom 105 is provided between an upper portion of the boom 105 and an upper portion of the mast 106; a second guy line 108 is provided between the upper portion of the mast 106 and a rear portion of the crane body 101; a counterweight car 121 is vertically rockably connected to the rear portion of the crane body 101 vertically rockably through a connecting arm 123; and a third guy line 109 is provided between the counterweight car 121 and the upper portion of the mast 106. When a hanging load W acts on the mast 106 from the boom 105 through the first guy line 107, the third guy line 109 is stretched and the weight of the counterweight car 121 resists the hanging load W. When a larger hanging load W acts on the mast to raise the counterweight car 121, the second guy line 108 is also stretched, so that not only the weight of the counterweight 121 but also the weight of the crane body 101 resists the load W.
In such structure, however, when the weight of the crane body 101 also comes to serve as resistance under the action of the large hanging load W, the resisting force from the crane body 101 acts on the mast 106 through the second guy line 108 whose angle relative to the mast 106 is comparatively small, as will be discussed hereinafter , so that a compressive load applied to the mast 106 increases remarkably, thus requiring the mast 106 to have a high strength. Further, the counter weight car 121 is desired to function so as to prevent the crane from falling sideways, but in the above structure the connection between the crane body 101 and the counterweight car 121 is merely through the connecting arm 123, that is, the connection strength in a torsional direction is poor, so this portion will be damaged under action of a force exerted in a sideways falling direction of the crane, and thus it is impossible to prevent the crane from falling sideways.
This type of crane can also be used as an ordinary crane by removing the counterweight car 121. In this connection, it is desired to facilitate rearrangement between the counterbalancing type crane using the counterweight car 121 and an ordinary type crane. In this case, the above conventional structure involves the problem of how to handle the guy line 109 disposed between the upper portion of the mast 106 and the --counterweight car 121. More particularly, when rearranging from the counterbalancing type crane into an ordinary type crane, the third guy line 109 becomes unnecessary. A lower end of the guy line 109 is removed from the -counterweight car 121, but if it is then attached to the crane body 101, it may contact and interfere with another rope or object. To avoid this, it is necessary to bring the mast 106 down to the ground and remove the third guy line 109 from the mast. This operation has been found to be very troublesome.